Try adding an endpoint for it

clientapi/routing/admin.go

@@ -90,6 +90,37 @@ func AdminEvacuateUser(req *http.Request, cfg *config.ClientAPI, device *userapi
 	}
 }
 
+func AdminPurgeRoom(req *http.Request, cfg *config.ClientAPI, device *userapi.Device, rsAPI roomserverAPI.ClientRoomserverAPI) util.JSONResponse {
+	vars, err := httputil.URLDecodeMapValues(mux.Vars(req))
+	if err != nil {
+		return util.ErrorResponse(err)
+	}
+	roomID, ok := vars["roomID"]
+	if !ok {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.MissingArgument("Expecting room ID."),
+		}
+	}
+	res := &roomserverAPI.PerformAdminPurgeRoomResponse{}
+	if err := rsAPI.PerformAdminPurgeRoom(
+		req.Context(),
+		&roomserverAPI.PerformAdminPurgeRoomRequest{
+			RoomID: roomID,
+		},
+		res,
+	); err != nil {
+		return util.ErrorResponse(err)
+	}
+	if err := res.Error; err != nil {
+		return err.JSONResponse()
+	}
+	return util.JSONResponse{
+		Code: 200,
+		JSON: res,
+	}
+}
+
 func AdminResetPassword(req *http.Request, cfg *config.ClientAPI, device *userapi.Device, userAPI userapi.ClientUserAPI) util.JSONResponse {
 	vars, err := httputil.URLDecodeMapValues(mux.Vars(req))
 	if err != nil {

clientapi/routing/routing.go

@@ -155,6 +155,12 @@ func Setup(
 		}),
 	).Methods(http.MethodGet, http.MethodOptions)
 
+	dendriteAdminRouter.Handle("/admin/purgeRoom/{roomID}",
+		httputil.MakeAdminAPI("admin_purge_room", userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {
+			return AdminPurgeRoom(req, cfg, device, rsAPI)
+		}),
+	).Methods(http.MethodGet, http.MethodOptions)
+
 	dendriteAdminRouter.Handle("/admin/resetPassword/{localpart}",
 		httputil.MakeAdminAPI("admin_reset_password", userAPI, func(req *http.Request, device *userapi.Device) util.JSONResponse {
 			return AdminResetPassword(req, cfg, device, userAPI)

roomserver/api/api.go

@@ -150,6 +150,7 @@ type ClientRoomserverAPI interface {
 	PerformRoomUpgrade(ctx context.Context, req *PerformRoomUpgradeRequest, resp *PerformRoomUpgradeResponse) error
 	PerformAdminEvacuateRoom(ctx context.Context, req *PerformAdminEvacuateRoomRequest, res *PerformAdminEvacuateRoomResponse) error
 	PerformAdminEvacuateUser(ctx context.Context, req *PerformAdminEvacuateUserRequest, res *PerformAdminEvacuateUserResponse) error
+	PerformAdminPurgeRoom(ctx context.Context, req *PerformAdminPurgeRoomRequest, res *PerformAdminPurgeRoomResponse) error
 	PerformPeek(ctx context.Context, req *PerformPeekRequest, res *PerformPeekResponse) error
 	PerformUnpeek(ctx context.Context, req *PerformUnpeekRequest, res *PerformUnpeekResponse) error
 	PerformInvite(ctx context.Context, req *PerformInviteRequest, res *PerformInviteResponse) error

roomserver/api/perform.go

@@ -234,3 +234,11 @@ type PerformAdminEvacuateUserResponse struct {
 	Affected []string `json:"affected"`
 	Error    *PerformError
 }
+
+type PerformAdminPurgeRoomRequest struct {
+	RoomID string `json:"room_id"`
+}
+
+type PerformAdminPurgeRoomResponse struct {
+	Error *PerformError `json:"error,omitempty"`
+}

roomserver/internal/perform/perform_admin.go

@@ -231,3 +231,27 @@ func (r *Admin) PerformAdminEvacuateUser(
 	}
 	return nil
 }
+
+func (r *Admin) PerformAdminPurgeRoom(
+	ctx context.Context,
+	req *api.PerformAdminPurgeRoomRequest,
+	res *api.PerformAdminPurgeRoomResponse,
+) error {
+	if _, _, err := gomatrixserverlib.SplitID('!', req.RoomID); err != nil {
+		res.Error = &api.PerformError{
+			Code: api.PerformErrorBadRequest,
+			Msg:  fmt.Sprintf("Malformed room ID: %s", err),
+		}
+		return nil
+	}
+
+	if err := r.DB.PurgeRoom(ctx, req.RoomID); err != nil {
+		res.Error = &api.PerformError{
+			Code: api.PerformErrorBadRequest,
+			Msg:  err.Error(),
+		}
+		return nil
+	}
+
+	return nil
+}

roomserver/inthttp/client.go

@@ -39,6 +39,7 @@ const (
 	RoomserverPerformForgetPath            = "/roomserver/performForget"
 	RoomserverPerformAdminEvacuateRoomPath = "/roomserver/performAdminEvacuateRoom"
 	RoomserverPerformAdminEvacuateUserPath = "/roomserver/performAdminEvacuateUser"
+	RoomserverPerformAdminPurgeRoomPath    = "/roomserver/performAdminPurgeRoom"
 
 	// Query operations
 	RoomserverQueryLatestEventsAndStatePath    = "/roomserver/queryLatestEventsAndState"
@@ -272,6 +273,17 @@ func (h *httpRoomserverInternalAPI) PerformAdminEvacuateUser(
 	)
 }
 
+func (h *httpRoomserverInternalAPI) PerformAdminPurgeRoom(
+	ctx context.Context,
+	request *api.PerformAdminPurgeRoomRequest,
+	response *api.PerformAdminPurgeRoomResponse,
+) error {
+	return httputil.CallInternalRPCAPI(
+		"PerformAdminPurgeRoom", h.roomserverURL+RoomserverPerformAdminPurgeRoomPath,
+		h.httpClient, ctx, request, response,
+	)
+}
+
 // QueryLatestEventsAndState implements RoomserverQueryAPI
 func (h *httpRoomserverInternalAPI) QueryLatestEventsAndState(
 	ctx context.Context,

roomserver/inthttp/server.go

@@ -65,6 +65,11 @@ func AddRoutes(r api.RoomserverInternalAPI, internalAPIMux *mux.Router) {
 		httputil.MakeInternalRPCAPI("RoomserverPerformAdminEvacuateUser", r.PerformAdminEvacuateUser),
 	)
 
+	internalAPIMux.Handle(
+		RoomserverPerformAdminPurgeRoomPath,
+		httputil.MakeInternalRPCAPI("RoomserverPerformAdminPurgeRoom", r.PerformAdminPurgeRoom),
+	)
+
 	internalAPIMux.Handle(
 		RoomserverQueryPublishedRoomsPath,
 		httputil.MakeInternalRPCAPI("RoomserverQueryPublishedRooms", r.QueryPublishedRooms),

roomserver/storage/interface.go

@@ -170,4 +170,5 @@ type Database interface {
 	ForgetRoom(ctx context.Context, userID, roomID string, forget bool) error
 
 	GetHistoryVisibilityState(ctx context.Context, roomInfo *types.RoomInfo, eventID string, domain string) ([]*gomatrixserverlib.Event, error)
+	PurgeRoom(ctx context.Context, roomID string) error
 }